For winter sports enthusiasts, hand warmers can mean the difference between calling it a day early and playing outside for as long as possible. In fact, anyone who braves cold temperatures might be tempted to try the little pouches that emit warmth within seconds of being exposed to air.

Hand warmers date back centuries to when the Japanese would use hot stones to warm their hands, says Keiko Ishikawa, a marketing manager of hand-warmer maker Mycoal USA. Portable hand warmers filled with hot ash were the version that followed, she says.

These days, disposable hand warmers turn up the heat in your mittens by means of an exothermic reaction that, in essence, just creates rust. Each pouch typically contains iron powder, salt, water, an absorbent material, and activated carbon. When the pouch is removed from its outer packaging, oxygen drifts across the pouch’s permeable covering. With salt and water present, the oxygen reacts with the iron powder located inside to form iron oxide (Fe2O3) and release heat.

The absorbent material can be vermiculite, pulverized wood, or a superabsorbent polymer such as polyacrylate. It helps retain the moisture so that the reaction can occur. The activated carbon helps to evenly disperse the heat produced, which can average 135 °F.

Although the chemistry of disposable hand warmers is simple, their engineering is more complicated. “You want to make this thing act quickly because people like to open up the packet and feel warm right away, but you also want it to last a long time,” says Joe Vergona, manager of engineering and product development for Grabber Performance Group, a Grand Rapids, Mich.-based company that sells hand warmers. For example, some hand warmers last seven hours, and others can last more than 24 hours.

To lengthen the time a hand warmer lasts, some companies opt to increase the amount of iron in the packet, Vergona says.

Another strategy is to experiment with the iron powder. “If you vary the raw materials in the warmer, you can change how quickly the reaction happens or how much of the warmer is reacted at one time,” Vergona adds. For example, the greater the surface area of the iron, the more it can react with oxygen to produce heat, he says.

Credit: Amanda Yarnell/C&EN

Credit: Amanda Yarnell/C&EN

The pouch material also affects the performance of the hand warmers. “It’s a balance of the ingredients inside the pack and the performance characteristics of the pouch itself,” Vergona says. The iron powder and other ingredients are contained in a blended nonwoven material that has specific permeability characteristics. If the pouch admits more oxygen, the reaction occurs more quickly. Toe warmers, for example, use a nonwoven material that lets in more oxygen to compensate for the low-oxygen environment inside a shoe. “The level of perforation, the size of the holes—all that’s going to govern how much oxygen enters the warmer,” Vergona says.

To extend the shelf life of hand warmers, the outside wrapper is specially chosen to ensure that minimal amounts of oxygen get in and minimal water gets out. “Any old plastic, and the hand warmers will last a week and die,” because oxygen can get in and spoil the product, Vergona says. The outside wrappers are usually made of polymers such as the plastic polyethylene.

The main difference between disposable hand warmers and some reusable versions is the chemicals used to produce the heat-releasing reaction. Reusable hand warmers don’t contain iron but instead use a supersaturated solution of sodium acetate that releases heat as it crystallizes. Boiling the used packet restores the solution to its supersaturated state. Air-activated hand warmers can’t be reused.

Besides warming hands and feet, the technology has other applications. For example, Grabber sells heavy-duty warmers that can be used to transport tropical fish. The company is also expanding into the medical and therapeutic fields.

Hand warmers can even be used to teach exothermic reactions, as Kathy Ceceri, a mom who homeschools her children, discovered. The packaging instructions might have “said not to open the packet, but I immediately opened it and poured the contents into a glass jar,” she says, noting that the iron powder began to smoke. By completely exposing the iron to air, she accelerated the exothermic reaction.

Ceceri—and anyone else who’s used hand warmers—has discovered that there’s nothing like simple chemistry to help turn up the heat.

i used hand warmers for golf and that very same day I noticed micro slivers in both my hands I have tried to remove them but have not been able to do so. They are very painful when brushed against anything. I have used a magnifying glass to see the slivers but can't see anything.. Have you heard of this happening??

I've used hand and toewarmers a great deal up here in the Northeast. Sometimes you get a bad batch and its probably because oxygen got in. It occured to me that it might be fun to try to make your own - as a science experiment, not to save money!

I was wondering if anyone knows the amount of iron in a single pouch. My dog got into one and I was wondering if the amount in one pack is enuf to kill him or if he will be ok. He definately did not eat it all, there was a huge mess left to clean up, if he ate any. Im not sure.Just need a starting point so I know if I should worry or not. thanks.